1. Field of the Invention
The present invention relates to an antenna system having retrodirective characteristics in which beams of radio waves to be transmitted are automatically radiated in the direction of arrival of incoming radio waves.
2. Description of the Prior Art
In FIG. 1, which represents a block diagram of a conventional antenna system, the reference symbols 1a to 1d represent transmitting/receiving element antennas; 2a to 2d duplexers for separating transmitted radio waves and received radio waves; 13 the direction of arrival of incoming radio waves; 14 an equiphase front (equiphase wave surface) of the incoming radio waves arriving from the direction of arrival 13 of the incoming radio waves received when the element antenna 1a is the reference; (15a) to (15d) distances from the equiphase front 14 to each of the element antennas 1a to 1d; and 16a through 16d represent amplifiers for amplifying the incoming radio waves and for providing the radio waves to be transmitted.
A description will now be given of the operation. The incoming radio waves received from the direction of arrival 13 thereof are received by the element antenna 1a and are delivered via the duplexer 2a to the amplifier 16b. The incoming radio waves are amplified by the amplifier 16b, and then provided as radio waves to be transmitted. The thus formed radio waves to be transmitted travel through the duplexer 2d and are then transmitted from the element antenna 1d. Similarly, the incoming radio waves received by the element antenna 1c travel via the duplexer 2b, the amplifier 16d and the duplexer 2c, and are then transmitted from the element antenna 1c. The incoming radio waves received by the element antenna 1c pass through the duplexer 2c, the amplifier 16c and the duplexer 2b, and are transmitted from the element antenna 1b. The incoming radio waves received by the element antenna 1d travel via the duplexer 2d and the amplifier 16a, and are radiated from the element antenna 1a through duplexer 2a. The electrical characteristics of each system are substantially the same, and the element antennas 1a through 1d are arranged in symmetry with respect to the central line 21. Hence, distances of arrival (15a) to (15d) from the equiphase front 14 based on the element antenna 1a to the individual element antennas 1a to 1d are expressed by the following formulae: EQU (15a)[=0]+(15d)=(15d) (1) EQU (15b)+(15c)=(15d) (2)
When the incoming radio waves travel the arrival distance (15d) from the equiphase front 14 and are received by the element antenna 1d and then transmitted from the element antenna 1a, the delay in each system is ignored here because all the electrical characteristics of the respective systems are substantially equal to each other, and the radio waves received by the element antenna 1a and transmitted from the element antenna 1d travel a distance corresponding to the arrival distance (15d) from the element antenna 1d [(15a)=0]. Similarly, the radio waves which are received by the element antenna 1b and transmitted from the element antenna 1c travel the arrival distance (15d) from the equiphase front 14 during the same time period. It is therefore apparent from the formula 2 that the radio waves have travelled the distance corresponding to the arriving distance (15c) from the element antenna 1c. At the same time, the radio waves which are received by the element antenna 1c and transmitted from the element antenna 1b travel the arrival distance (15d) from the equiphase front 14. Hence, it follows from the formula 2 that the radio waves reach a position spaced apart from the element antenna 1b by the arrival distance (15b).
The situation with respect to the equiphase front of the radio waves to be transmitted is the same as the equiphase front 14 of the incoming radio waves arriving from the direction of arrival thereof. Namely, the radio waves transmitted travel the arrival distance (15a)[=0] from the element antenna 1a; the arrival distance (15b) from the element antenna 1b; the arrival distance (15c) from the element antenna 1c; and the arrival distance (15d) from the element antenna 1d. Hence, the transmitted beams can be automatically radiated in the same direction as the direction of arrival 13 of the radio waves.
In the thus arranged conventional antenna system, all the electrical characteristics of each part of the system are required to be equal to each other. Therefore, the frequency of the transmitted radio waves has to be equalized with that of the incoming radio waves. A problem arises, however, in that the incoming radio waves interfere with the transmitted ones if the frequencies thereof are equalized. In order to obviate this problem, it is required that the frequency of the transmitted radio waves is different from that of the incoming radio waves. However, the electrical characteristics in the receiving mode differ from those in the transmitting mode. As a result, another problem occurs because the beams of transmitted radio waves cannot be automatically radiated in the direction of arrival 13 of the incoming radio waves, since the equiphase front of the incoming radio waves arriving in the direction of arrival 13 thereof does not coincide with the equiphase front of the transmitted radio waves.